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<head>
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    <title>Document</title>
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<body>
    
    <script>
        //得到手机屏幕的宽度
        let htmlWidth = document.documentElement.clientWidth || document.body.clientWidth;
        //得到html的Dom元素
        let htmlDom = document.getElementsByTagName('html')[0];
        //设置根元素字体大小
        htmlDom.style.fontSize = htmlWidth / 20 + 'px';
        if(htmlWidth>750){
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